A full description of the indices Kn, Ks, Km is given in a monography,"Indices Kn,
Ks et Km, 1964-1967", edited in 1968 by the Centre National de la Recherche Scientifique,
15 quai Anatole, France, 75007 Paris, which contains these indices for 2964-1967 while
the International Association of Geomagnetism and Aeronony (IAGA) Bulletin NO. 39
contains those for 1959-1963. Yearly computations of these data are published in the
series of IAGA Bulletin No.32. All of them are available in digital form at the appropriate
World Data Center. Briefly, the three-hourly indices Kn and Ks for the Northern and
Southern Hemispheres respectively are derived from the K indices of observatories
approximately well distributed in latitude and in longitude. The indices are standardized
according to the distances of the stations to the auroral zones. The stations are
arranged in groups representing a longitude sector in one of the hemispheres (5 in
the Northern Hemisphere, 4 in the Southern). The observatories currently in use are:
MGD-Magadan, EYR-Eyrewell, PET-Petropavlovsk, LAU-Lauder, MMB-Memambetsu, CAN-Canberra,
POD-Podkammenaya Tunguska, SVE-Sverdlovsk, GNA-Gnangara, NGK-Niemegk, KGL-Kerguelen,
CZT-Crozet, HAD-Hartland, HER-Hermanus, OTT-Ottawa, AIA-Argentine Island, FRD-Fredericksburg,
NEW-Newport, VIC-Victoria, TUC-Tucson The mean standardized K of each sector is converted
into an equivalent amplitude and the weighted (in longitude) averages and these amplitudes
are converted back into Kn and Ks. Km is derived in the same way from am, the average
of an and as. Indices an, as and am are expressed in gammas (one gamma equals one
nanoTesla) and correspond to the magnetic activity level (as it can be inferred from
K indices 0 at an invariant magnetic latitude of 50 degrees. Indices Kn, Ks and Km
are expressed in the same units as Kp. Values published in these reports are only
provisional because in some months all observatories used in each longitude sector
have not sent K indices at the right time and because K indices of Antarctic Stations
have be rescaled at the end of each wintering.